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Virtually everyone knows marijuana use in the United States has skyrocketed in recent years thanks to the loosening of state laws that had limited the drug to medicinal and related purposes. But by just how much has been anybody's guess.

A new survey of more than 500,000 adults now puts a number on the change - and it's big. From 2002 to 2014, the percentage of adults using marijuana jumped from 10.4 per cent to 13.3 per cent. Those using it daily or close to that went from 1.9 percent to 3.5 per cent. That means there could be 31.9 million adults using marijuana - with 8.4 million of them using it a lot.

The results, reported in Wednesday's The Lancet Psychiatry, also noted an important trend in how marijuana is regarded. Although the drug has "become increasingly potent over the past decade," the authors wrote, fewer people think it's harmful.

Wilson M. Compton, a researcher with the National Institute on Drug Abuse who worked on the study, described this shifting perception as a worrisome development and said it suggests a need for improved education on the risks.

"Understanding patterns of marijuana use and dependence and how these have changed over time is essential for policymakers who continue to consider whether and how to modify laws related to marijuana and for health-care practitioners who care for patients using marijuana," he explained.

That said, the data from the National Survey on Drug Abuse and Health did offer a positive finding: The prevalence of marijuana abuse or dependence remained stable at about 1.5 per cent over the period.

Marijuana has both short- and long-term effects on the brain and physical health. Scientists are still learning more about the different components of the cannabis plant and how they impact the human body, but there's consensus that smoking marijuana may irritate breathing passages, that it increases the heart rate and that it may negatively impact a fetus if a woman smokes while pregnant.

On the flip side, many people swear by it for chronic pain. There's also evidence that it may help with nausea, sleep disorders, depressed appetite and a number of other conditions.

A lot of the positive publicity around marijuana's medicinal use has recently focused on how one extract made from cannabis may benefit some patients with epilepsy. In March, GW Pharmaceuticals said its studies showed that its cannabis-based drug appeared to dramatically reduce seizures in patients with Dravet syndrome.

The Drug Enforcement Administration said in August that while it would still maintain marijuana on the list of the most dangerous drugs, it would loosen research restrictions to make it easier for scientists to look into other possible medical benefits - a move almost universally applauded, even by the strictest opponents of national marijuana legalization.